The actual water heater itself is the same. Which one you choose depends how you'll be using it.
HOTTAP - If you'll always be using the HOTTAP with pressurised water (mains water, house pump, caravan park, water tap, your own 12v pump, eg), then all you need is the HOTTAP.
HOTTAP OUTING - If you'll be using this in areas where there is no pressurised water (off-grid shack, free camping, on the beach, shearing shed, middle of the outback, fishing in the high country, etc) then you'll need the HOTTAP OUTING.
The HOTTAP OUTING is a complete system which includes our 12V pump (along with all the necessary bits and pieces needed to connect our pump to a 12v battery and the inlet and outlet water hoses etc).
The HOTTAP OUTING gives you complete flexibility. If you want to use it at home where you have mains water, simply connect the garden tap to it. Going free camping for a week? Make sure to take your 12v pump and a water bucket and you'll have hot water for the whole week.